The current global concern for the environment has generated a strong need by both government and industry for technologies that reduce emissions of NO.sub.x. NO.sub.x are primary contributors to photochemical smog and acid rain, and may deplete the ozone layer. Up to millions of tons of nitrogen oxides, generally denoted herein as "NO.sub.x," are emitted into the atmosphere each year as a result of numerous industrial and military processes, ranging from high temperature combustion of fossil fuels, to explosive manufacturing and munitions disposal processing and further to operations of powered aerospace ground equipment.
The impact of such emissions on human health and the environment in general has been the subject of intense study and public debate and legislative action to mandate safer emissions has already been enacted. For example, the Clean Air Act Amendment of 1990 mandates that emission generating industrial plants develop and/or implement techniques to significantly reduce their emissions of NO.sub.x. Such legislation affects power plants, iron and steel plants, pulp and paper mills, acid production plants, petroleum refineries, lime plants, fuel conversion plants, glass fiber processing plants, charcoal production plants, cement plants, copper smelters, coal cleaning plants, etc.
Developmental efforts have been directed to converting NO.sub.x to the individual elemental diatoms, N.sub.2 and O.sub.2. Conventional processes typically utilized thermal techniques for generating very high temperature conditions within a reactor. These techniques are highly inefficient as excessively high electrical power is needed not only to treat relatively low pollutant concentrations, but to cool the resultant effluent emerging from the reactor.
Electron beam irradiation has also been used in various forms to convert to the individual elemental diatoms. Such systems also use electron beams or ultraviolet light to oxidize the NO.sub.x. The ionization caused by the electron beam irradiation converts the NO.sub.x to acid mist at low temperatures and/or solid particles at high temperatures which may be removed by conventional methods employing filters and scrubbers. However, due to potential harm to operation personnel, costly and elaborate shielding measures must be employed.
Numerous research agencies have investigated the use of non-thermal plasma devices ("NTPDs") to reduce NO.sub.x in gas streams. These devices employ electrodes and dielectrics driven by a voltage supply for generating an electric field driving electrochemical reactions converting NO.sub.x to other atomic and molecular forms, either chemically less toxic, and/or structurally more readily removed from the gas stream. Numerous materials have been used to construct such dielectrics, including quartz, glass, alumina, mullite, and oxide free ceramic such as silicon nitrite, boron nitrite, aluminum nitrite.
Of the conventional non-thermal plasma devices, the dielectrics isolating the electrodes from the gas and enabling the non-thermal plasma environment which drives various electrochemical reactions are typically constructed of glass. While glass has the required thermal and electrical properties, it fractures easily and may not be well suited for any mobile applications of non-thermal plasma devices. In addition, an appreciable thickness of glass is typically used to standoff the voltages applied, which limits the flow rate or throughput of the devices. As for the other conventional dielectric materials, many have suitable dielectric strength and many have suitable working temperatures; however, none combine both of these factors in providing a desirable dielectric for use in an efficient effluent gas treating non-thermal plasma apparatus.
Accordingly, there exists a demand for an improved non-thermal plasma apparatus for treating NO.sub.x bearing gas streams, with significantly improved efficiency and durability. It is also desired that such an apparatus has significantly increased applicability and ease of application. In accordance with the present invention, a non-thermal plasma apparatus is provided, having a plasma reactor and an inlet and outlet connected thereto through which the exhaust gas enters and leaves the plasma reactor. A scrubber may be provided to scrub the exhaust gas leaving the plasma reactor and a stack may be connected to the scrubber to permit the exhaust gas to exit the apparatus. The plasma reactor is equipped with a plurality of dielectrically-coated electrodes between which a selected voltage is applied to generate a non-thermal plasma environment for driving selected electrochemical reactions. A voltage supply is electrically configured to apply a predefined voltage across the electrodes to create microdischarges in the exhaust gags stream. Where such electrochemical reactions involve the conversion of nitric oxides to various diatomic and molecular forms, including primarily nitrogen dioxide, the efficiency (in terms of energy per molecule of remediated NO.sub.x) and total reduction (in terms of percentage of hazardous compounds reduced per treatment pass) may be increased by approximately 30% when compared with conventional non-thermal plasma devices.
Operating with improved efficiency and durability, the non-thermal plasma apparatus utilizes the dielectrically-coated electrodes of the plasma reactor to drive the desired conversions. In that regard, the electrodes are constructed of metal plates that are coated with a fluoropolymeric material, such as fluorocarbon. Depending on the level of contamination in the exhaust gas and the flow rate of the exhaust gas, a selected thickness of fluorocarbon coating is applied to the planar surfaces of the plate, forming "double-dielectric" electrodes. In one embodiment, a plurality of such double-dielectric electrodes are used in the plasma reactor, arranged in parallel formation and alternating between positive and negative charges. Fluoropolymeric (e.g., fluorocarbon) spacers are positioned between adjacent double-dielectric electrodes and may be configured with selected thicknesses to provide a plurality of desired reaction zones or gaps therebetween. With a plurality of gaps, the total non-thermal plasma environment is expanded to increase the overall flow rate of the exhaust gas or throughput of the apparatus.
In another embodiment, a plurality of double-dielectric electrodes may be supported on a specially configured fluoropolymeric (e.g., fluorocarbon) insulators adapted for placement inside the plasma reactor. In particular, the insulators may be configured with grooves or indentations which support the double-dielectric electrodes at their edge portions, in parallel formation, alternating between positive and negative charges. The grooves may be configured to be spaced apart at selected distances such that adjacent double-dielectric electrodes may provide a plurality of desired reaction zones or gaps. Again, with a plurality of gaps, the total non-thermal plasma environment is expanded to increase the overall flow rate of the exhaust gas or throughput of the apparatus.
The non-thermal plasma apparatus of the instant invention may provide for the pretreatment of the exhaust gas with ethanol, either by vapor absorption or direct vapor injection. In particular, before the exhaust gas is exposed to plasma reactor, at least a portion of the exhaust gas is exposed to ethanol before entering the plasma reactor. One method involves diverting at least a portion of the exhaust gas through an ethanol bath before the exhaust gas enters the plasma reactor. Another method involves directly injecting ethanol directly into the path of the exhaust gas before it enters the plasma reactor.
For either method, the high vapor pressure of ethanol permits a significant portion of the ethanol to be absorbed by the exhaust gas. As such, the ethanol-bearing exhaust gas is pretreated in preparation for the plasma reactor. In particular, the plasma reactor exposes to the pretreated exhaust gas to reactive species generated by the plasma reactor, e.g., oxygen atoms, whereupon nitric oxides are converted to a variety of products, including primarily nitrogen dioxide, with significantly improved efficiency. As an added advantage, the solubility of ethanol permits the ethanol to be readily scrubbed from the exhaust gas downstream of the plasma reactor, along with the converted nitrogen dioxide.
To implement the ethanol pretreatment, the non-thermal plasma apparatus comprises an inlet and an outlet connected to the reaction chamber, permitting the exhaust gas to enter and leave the reaction chamber. In one embodiment, the inlet is further connected to a diverter equipped with an injector, which diverts a portion of the gas stream through an ethanol bath before reinjecting the ethanol-bearing gas stream into the inlet. Ethanol is readily absorbed by the gas stream as it passes through the ethanol bath.
In another embodiment, the inlet is equipped with an injector which receives a supply of ethanol that is sprayed as a fine mist directly into the gas stream. The fine mist of ethanol is substantially uniformly absorbed by the gas stream before it enters the reactor chamber. A reservoir stores the ethanol which is delivered to the injector by a metered pump.
The non-thermal plasma treatment may be used for a variety of gas streams. A typical gas stream contains approximately nitrogen, oxygen, water vapor and nitric oxide. The primary function of the treatment is to convert the nitric oxide into nitrogen dioxide.
These, as well as other features of the invention, will become apparent from the detailed description which follows, considered together with the appended drawings.